The present invention relates generally to aircraft gas turbine engine augmenters and, more specifically, to augmenter pilots.
High performance military aircraft typically include a turbofan gas turbine engine having an afterburner or augmenter for providing additional thrust when desired. The turbofan engine includes, in serial flow communication, a multistage fan, a multistage compressor, a combustor, a high pressure turbine powering the compressor, and a low pressure turbine powering the fan. During operation, air is compressed in turn through the fan and compressor and mixed with fuel in the combustor and ignited for generating hot combustion gases which flow downstream through the turbine stages which extract energy therefrom. The hot core gases are then discharged into an augmenter from which they are discharged from the engine through a variable area exhaust nozzle.
The augmenter includes an exhaust casing and liner therein which defines a combustion zone. Fuel spraybars and flameholders are mounted between the turbines and the exhaust liner for injecting additional fuel when desired during reheat, thrust augmentation, or afterburning operation for burning in the augmenter combustor for producing additional thrust.
In a bypass turbofan engine, an annular bypass duct extends from the fan to the augmenter for bypassing a portion of the fan air around the core engine to the augmenter which bypass air is used in part for cooling the exhaust liner. The bypass air must also be efficiently mixed with the core gases prior to discharge through the exhaust nozzle. This is typically accomplished using a convoluted daisy mixer having circumferentially alternating hot and cold chutes which respectively channel the core gases and bypass air therethrough in radially outward and inward paths for mixing at the exit plane thereof.
Various types of flameholders are known and typically include radial and circumferential V-shaped gutters which provide stagnation regions there behind of local low velocity regions in the otherwise high velocity core gases for sustaining combustion during reheat operation. Augmenter operation includes fuel injection into an augmenter combustion zone and ignition is initiated by some type of spark discharge or other igniter or auto-ignition dues to hot core gases. Since the rate of gas flow through an augmenter is normally much greater than the rate of flame propagation in the flowing gas, some means for stabilizing the flame is usually provided, else the flame will simply blow out the rear of the engine, and new fuel being injected will not be ignited.
Typically, flameholders are used for flame stabilization. In regions where the gas flow is partially recirculated and the velocity is less than the rate of flame propagation, there will be a stable flame existing which can ignite new fuel as it passes. Unfortunately, flameholders in the gas stream inherently cause flow losses and reduced engine efficiency.
Since the core gases are the product of combustion in the core engine, they are initially hot, and are further heated when combusted with the bypass air and additional fuel during reheat operation. The mixing of the core gases with the bypass air results in cooling of the core gases. Augmenters require an ignition source in the cases where the core gas conditions are not conducive to reliable auto-ignition so pilots are used to start and maintain combustion in the afterburner. One potential problem with an afterburner is that, at some flight conditions, its pilot stage may not light due to an excessively lean fuel-air ratio in the vicinity of the igniters. A second problem is that the time in an operating pilot stage may blow out when the aircraft fuel system supplies fuel to pilot spray rings or bars. This latter problem occurs because the fuel pressure in the pilot spray ring momentarily diminishes as the aircraft fuel system initially attempts to supply both the pilot spray ring and the auxiliary spray rings. As a result, the fuel-air ratio becomes too lean to sustain combustion of the pilot flame.
Thus, it is highly desirable to have an augmenter or afterburner that can produce a stable flame and have reliable ignition for igniting thrust augmenting fuel while holding down flow losses and attendant reductions in engine performance and fuel efficiency.